Electronic devices employ electrical circuits implemented as one or more integrated circuits (ICs) for numerous applications. For example, ICs are configured to achieve desired functions, for example, control of associated devices, digital-to-analog (D/A) or analog-to-digital (A/D) conversion, mixed signal analysis, etc.
An IC receives electrical energy from an external power supply at one or more input terminals to provide operating power for performing desired functionality. In some IC configurations, a power device may be used to isolate the external supply from another internal or external node. This isolated node becomes a temporary power source for the IC when the external supply becomes low or is disconnected. This node is usually coupled to internal circuitry in an IC.
For example, in some ICs that drive motors, the isolated node is normally supplied with power from an external supply. The power flows through the isolation device to the isolation node under these normal conditions. When the external supply is too low or unavailable, the isolation device can be turned off and the isolated node is supplied with power from the spinning motor. Many types of power devices, such as Field Effect Transistors (FETs), have a parasitic diode that is an inherent part of their construction. The polarity of this diode is arranged so that it is reverse biased when the external supply voltage is lower than the isolation node voltage. An unexpected short condition at the terminal of such an isolated node can cause undesirable conditions within the isolation device. For example, if the terminal is shorted to ground potential, electrical current can flow from the external power supply to the isolated node because the parasitic diode is forward biased. This current can become quite large which, in turn, can adversely affect the isolation device. If the isolation device is part of an IC, the current can adversely affect other circuitry within the IC and, in turn, associated external circuitry and equipment.
Various approaches have been proposed to help protect the power isolation device and associated circuitry during a short circuit condition, such as may occur at a terminal of an IC. For example, a Schottky diode can be connected between the power supply terminal and the isolation terminal. The Schottky diode is thus in parallel with, and has the same polarity as, the parasitic diode. In such an arrangement, most of the current during a short circuit condition would flow through the Schottky diode because it has a relatively low forward bias voltage compared to the forward bias voltage of the parasitic diode of a typical isolation device. However, if the Schottky diode is added to the IC as an off-chip component, it adds significantly to the overall cost of the resulting system. An on-chip Schottky diode requires a large amount of die area and, therefore, would increase die cost for the IC.
An alternative approach is to employ a fuse to operate during a short circuit condition. In this approach, it may be difficult to select a fuse that is capable of providing adequate protection during a short circuit, while permitting desired normal operation of the IC. This approach also has the drawback that normal operation cannot resume if the short is removed.
Another alternate approach is to employ back-to-back power isolation devices. In such an arrangement, two power devices are placed in series such that the associated parasitic diodes have opposite polarities. This approach also adds significantly to the overall cost of the resulting system because of the need for the additional power device.